An optical scanning device used in a copy machine, a printer and the like generally includes a light source, a polygon mirror (a rotating polygon mirror) for deflecting and scanning beam light from the light source in a main scanning direction, a fθlens for forming an image of the beam light deflected by the polygon mirror on a surface to be scanned, and one or more planar mirrors provided on optical paths between the polygon mirror and the surface to be scanned.
Herein, when a manufacturing error, a mounting error and the like occur in the fθlens or the planar mirror, there is a case of causing a phenomenon (hereinafter, referred to as scanning line curvature) that beam light is curved in a sub-scanning direction.
Therefore, for example, Patent Literature 1 discloses an optical writing device (an optical scanning device) that curves a reflection mirror (a planar mirror) in order to suppress scanning line curvature. In this optical writing device, a scanning line curve adjusting means is provided to adjust the curved amount of the reflection mirror in a direction curved in a convex and concave shape with respect to the incidence direction of light beams. In this way, even though scanning lines are curved in a convex shape to one side in a sub-scanning direction and are curved in a convex shape to the other side in the sub-scanning direction, the reflection mirror is curved in a concave shape or a convex shape with respect to the incidence direction of the light beams, so that it is possible to reduce the curved amount of the scanning lines.
In addition, in a color image forming apparatus, when scanning line curvature occurs on surfaces to be scanned of photoreceptors corresponding to each color, since a color shift occurs in a color image to be formed due to a shift of scanning lines, the scanning line curvature is likely to be problematic in the color image forming apparatus.